Ionization Dynamics in Intense Laser-Plasma Interaction

Recently, laser-accelerated ion beams from the back of
solid targets have been observed in petawatt laser experiments, showing
high levels have been observed in petawatt laser experiments, showing high
levels of beam intensity and low emittance. Experiments and previous numerical
modelling suggest that these ions are produced by sheath acceleration off
the rear surface of laser-irradiated targets; they are ionized and accelerated
by quasistatic electric fields that are due to an expanding cloud of hot
electrons coming from the front side of the targets.

We investigate the ionization dynamics in these targets by means of one-dimensional
particle-in-cell (1D-PIC) simulations including collisions, ie. collisional
ionization as well as binary collisions between particles, in addition
to the usual field ionization models [1]. Our aim is mainly to understand
from where in the target the different ion species, ie. charge states, originate
in order to manipulate ion beam spectra at an early stage.

We further investigate the subsequent acceleration of the ions, paying special
attention to the role of contaminants on the target's back surface. Our past
results show good agreement with experiments by Hegelich [2], giving
qualitatively identical features in the ion spectra. In the late acceleration
stage we find good agreement between PIC simulations and self-similar solutions
for the quasi-neutral expansion of plasma mixtures into vacuum [3].